Known examples of this type of compliance unit are disclosed in Patent Literatures 1 to 3. This compliance unit includes a first body and a second body, which are coupled to each other while having a degree of freedom with which the first body and the second body are displaceable relative to each other, and is used by attaching the first body to a robot hand and attaching a workpiece-holding unit to the second body. When a workpiece held by the workpiece-holding unit is inserted into an insertion hole of another workpiece set on a table so as to be coupled to the other workpiece, if there is a misalignment between the workpiece and the insertion hole, the compliance unit accommodates the misalignment by displacing the second body.
The compliance unit has a returning mechanism for causing the second body to return to a home position and restraining the second body at the position. This returning mechanism operates when the workpiece-holding unit holds and transports a workpiece to a coupling position and restrains the second body at the home position so as to prevent the workpiece from being displaced due to displacement of the second body and to prevent the workpiece from breaking as a result of being displaced.
However, the returning unit operates by using air, and thus, there has been a problem in that, if the air supply to the returning mechanism is shut off due to an unexpected cause during the period when the workpiece is transported by the workpiece-holding unit, the returning unit does not function, and the second body is displaced, which in turn results in positional deviation of the workpiece.